Overworld fix... hmm... I have never explored it, but it depends whether you need different biomes, or you just want different terrain. As I think with the vanilla superflat tweaker (thing you get when making a world) you can force it to look like different biomes (grass floor. sand floor, etc.)To be more specific, like i probably should have been in the first place, I'm making a modpack and I would like to make the map a superflat version of the overworld. It's going to be for 1.7.10, but it looks like Mystcraft hasn't updated yet and even then I need to use it for the overworld. That, and Mystcraft isn't going to be in the pack, so weirdness.
EDIT: While i'm here, and i can't seem to access the support site, can anyone tell me how to fix this?: http://pastebin.com/7WAKXXg9
As for the crash, I am not great with crashlogs that aren't from a "vanilla" launcher. But it looks like you are giving 1 gig of RAM... so I would assume it is that